A storage apparatus having multiple physical storage devices, and a storage system comprising one or more storage apparatuses are known. As a physical storage device, for example, there is a hard disk drive, a flash memory device, and so forth. The storage apparatus generally comprises a RAID group that conforms to RAID (Redundant Array of Independent (or Inexpensive) Disks) technology. The RAID group comprises multiple physical storage devices. Data is stored in the RAID group in accordance with a prescribed RAID level. Either one or multiple logical storage devices (logical volumes) are created based on the storage space of the RAID group. The logical volume is provided to a host computer coupled to the storage apparatus. The host computer sends the storage apparatus an access command (for example a write command or a read command) specifying the provided logical volume. Data access to this logical volume is carried out in accordance with this.
As the storage apparatus, a storage apparatus in which thin provisioning (may also be called dynamic provisioning) technology is used is known (Patent Literature 1). According to thin provisioning, a virtual volume (referred to as VVOL hereinafter), which is a virtual logical volume, is provided to the host computer instead of the fixed-storage-capacity logical volume. The VVOL is a set of multiple virtual areas (virtual storage areas). In a case where a write is generated to the VVOL from the host computer, a page is assigned from a pool to the virtual area. Furthermore, the pool is a storage area comprising multiple pages. The page is a real area (a substantial storage area), and, more specifically, for example, is a storage area obtained by using a LBA (Logical Block Address) to partition the logical volume, which comprises the pool. As used here, the “LBA” is an address used for specifying a location on a logical volume in a case where the host computer reads/writes data from/to the storage apparatus.
Furthermore, a pool in which multiple groups of pages with different access performance coexist is known (Patent Literature 2). For example, a high-performance page group comprises a high-performance logical volume, and a low-performance page group comprises a low-performance logical volume. The access performance of a logical volume depends on the access performance (and/or the RAID level of the RAID group) of the physical storage device that constitutes the basis of this logical volume. Reassignment (page reassignment hereinafter) can be carried out from a certain page inside a certain logical volume to a page inside a different logical volume.
Furthermore, among the logical volumes comprising the pool, a set of logical volumes having the same access performance is generally called a tier. In other words, the pool comprises multiple tiers, that is, multiple page groups with different access performance.
In Patent Literature 2, a tier to be assigned to each VVOL can be specified in accordance with the required performance of each VVOL. As used here, the required performance of the VVOL denotes the performance required with respect to this VVOL, for example, the throughput, response time, or other such access performance.
In accordance with this, for example, it is preferable to assign a high-performance tier to a VVOL with a high required performance, and to assign a low-performance tier to a VVOL having a low access frequency.
Patent Literature 3 discloses a method for deciding a storage capacity to be added to the pool in a case where the pages assigned to the VVOL are insufficient. In other words, Patent Literature 3 discloses a technology for regularly predicting a depletion of pool capacity, and preparing for the pool capacity depletion.